The present invention relates to a process for production of metallic powders such as those of nickel, copper, and silver which are suitable for various uses such as conductive paste fillers used for electrical parts for multi-layer ceramic capacitors, for titanium bonding materials, and for catalysts.
Conductive metallic powders such as those of nickel, copper, and silver are useful in internal electrodes in multi-layer ceramic capacitors. In particular, nickel powder has been researched, and especially ultrafine nickel powder produced by a dry production process is seen as being promising. Ultrafine powders having particle sizes of not only less than 1.0 xcexcm but also less than 0.5 xcexcm are in demand because of requirements for forming thin layers and for having low resistance in accordance with trends toward miniaturization and larger capacity in capacitors.
Various kinds of processes for production of metallic powders such as the above have been proposed. As one of the process for production of ultrafine spherical nickel powders having an average particle size in the range of 0.1 xcexcm to a few xcexcm, for instance, Japanese Patent Application, Second Publication No. 59-7765 (7765/84) discloses a process in which a solid mass of nickel chloride is heated and vaporized to form a vapor of nickel chloride, and then hydrogen gas is injected to the nickel chloride vapor at a high velocity, thereby causing a nuclear growth in unstable interface regions. Japanese Patent Application, First Publication, No. 4-365806 (365806/92) discloses a process in which the partial pressure of a vapor of nickel chloride obtained by vaporizing a solid mass of nickel chloride is set in the range of 0.05 to 0.3, and is reduced in a gaseous phase at a temperature ranging from 1004 to 1453xc2x0 C.
In the processes for production of metallic powders as proposed in the above, the reducing reaction is performed at a temperature of about 1000xc2x0 C. or more, so that the particles of the metallic powder which easily form secondary particles through agglomeration at temperatures in the temperature range for the reduction process and subsequent processes. As a result, a problem that the required ultrafine metallic powder cannot be reliably produced remains.
Therefore, an object of the present invention is to provide a process for production of metallic powder, in which the growth of particles in a metallic powders formed in a reduction process as secondary particles through agglomeration after a reduction process is suppressed, and a ultrafine metallic powder having a particle size of, for example, 1 xcexcm or less can be reliably produced.
During a process for production of metallic powder in a gaseous phase, metallic atoms are formed at the instant when a metallic chloride gas contacts a reductive gas, and ultrafine particles are formed and grow through collision and agglomeration of the atoms. The particle size of the formed metallic powder depends on conditions such as the partial pressure and the temperature of the metallic chloride gas in the atmosphere of the reduction process. After forming a metallic powder having a required particle size, the metallic powder is generally washed and recovered. Therefore, a cooling process for the metallic powder transferred from the reduction process is provided.
However, as the reduction process is performed at about 1000xc2x0 C. or at a temperature in a higher temperature range, the particles agglomerate again to form secondary particles while the powder is cooled from a temperature range for the reducing reaction to the temperature at which the growth of the particles stops, and therefore a metallic powder having required particle size cannot be reliably produced. Therefore, the inventors directed their attention to the rate of cooling in the cooling process, and studied the relationship between the cooling rate and the particle size of the metallic powder. As a result, they discovered that agglomeration of particles does not occur when the cooling is rapid, and in particular, that very ultrafine metallic powder can be produced when the powder is cooled at a cooling rate of 30xc2x0 C./sec or more from a temperature in the temperature range for the reducing reaction to a temperature of 800xc2x0C. or less.
The present invention was achieved based on the above research, and provides a process for production of metallic powder comprising contacting a metallic chloride gas with a reductive gas in a temperature range for a reducing reaction to form a metallic powder, and then contacting the metallic powder with an inert gas to cool the powder at a cooling rate of 30xc2x0 C./sec or more from the temperature range for the reducing reaction to a temperature of 800xc2x0 C. or less. According to the invention, agglomeration of the particles in the metallic powder after the reduction process is suppressed, and the particle size of the metallic powder formed in the reduction process is maintained. As a result, a metallic powder with required ultrafine particles can be reliably produced.